Power line communications technology aims to transmit digital data by exploiting the existing infrastructure of the electrical power grid. It notably allows the remote reading of electrical meters, the exchanges between electric vehicles, and the recharging terminals or else the management and the control power grids (smart grid). Power line communications (PLC) technology notably incorporates communication by narrow-band power line communications (or N-PLC) which is generally defined as a communication over an electrical power line operating at transmission frequencies up to 500 KHz.
N-PLC communication thus generally uses the bands of frequencies notably defined by the European electrotechnical standards committee (Comité Européen de Normalisation ELECtrotechnique—CENELEC) or by the Federal Communications Commission (FCC). Thus, if the CENELEC A band of frequencies (3-95 kHz) is considered, the transmission frequencies are situated between 42 and 89 KHz in the PRIME standard whereas they are situated between 35 and 91 KHz for the PLC-G3 standard.
In these frequency bands, the electrical cables carrying the signals by power line communications are in a very difficult environment. They are notably subjected to interference of the white noise, colored noise, or pulse noise type. Furthermore, they are not protected against any interference. For this reason, any FM/AM radio signal or any wireless communication can lead to the presence of harmonics of these signals within the useful frequency band used by narrow-band PLC communications.
Furthermore, the properties and characteristics of electrical power grids are not known a priori and are variable over time. Thus, interference may be created on an electrical power line when a user connects any given device such as for example a hair dryer or a washing machine. This then results in a propagation of intense frequency harmonics which may also be situated within the useful band of the PLC communications.
Accordingly, such noise signals, which are generally narrow-band noise signals (i.e. Narrow Band Interferer), in other words having a smaller frequency band than the frequency band of the useful signal, interfere with the synchronization phase of the receiver connected to the electrical power line, during which the receiver must be able to be synchronized in order to notably locate the start of the useful data of the frame of symbols carried by the useful signal. The thesis by Brian Michael Donlan titled “Ultra-wideband Narrowband Interference Cancellation and Channel Modeling for Communications”, 31 Jan. 2005, Blacksburg, Va., discloses various techniques for eliminating narrow-band noise signals from an ultra-wide-band (UWB) signal, in particular, in the context of spectrum spreading (Spread Spectrum). Some of the approaches disclosed in this document use predictive filters so as to estimate the noise signal before subtracting it from the received signal.
The signals mentioned in this document exhibit characteristics that are very different from the signals used in the communications using power line communications. Indeed, UWB signals (and, in particular, those using direct sequence spread spectrum) exhibit a spreading of the power of the transmitted signal over a wide band of frequencies in order to bury this power in the ambient noise or within the other communications. Thus, the power spectral density (or PSD) of a UWB signal is generally defined as being less than −41 dBm/MHz.
The signals used in PLC communications are signals modulated according to a multi-carrier modulation, for example, a modulation in quadrature on orthogonal carriers (i.e. an Orthogonal Frequency Division Multiplexing (OFDM) modulation), but using only a sub-set of carriers from amongst a larger set of available carriers. Thus, for example, if the CENELEC A band of frequencies is considered, the size of the inverse Fourier transform and of the direct Fourier transform is equal to 512, whereas only 97 sub-carriers (the sub-carriers 86 to 182) are used for the transmission in the PRIME standard. If the CENELEC A band of frequencies is considered, the size of the inverse Fourier transform and of the direct Fourier transform is equal to 256 whereas only 36 sub-carriers (the sub-carriers 23 to 58) are used in the PLC-G3 standard. Furthermore, it may be useful, during the synchronization phase, not to miss any symbol coming from the channel even when the latter is affected by noise.